This invention relates to inorganic microporous or ultrafiltration membranes and to a process for preparing such membranes.
Prior to the present invention, two basic processes have been available for the preparation of inorganic membranes. In both processes, insoluble inorganic particles are first deposited from a slurry onto a porous inorganic substrate. A layer or film of particles is formed on the substrate surface. The film is a porous structure resulting from the spaces between the film particles. Bilayers of different sized inorganic particles can also be used to form the film on a coarse substrate. The first bilayer comprises relatively large particles while the second layer comprises smaller particles that are too small to be deposited directly on the coarse substrate.
In one process, after the particles have been deposited on the substrate, the particles are first dried and then bonded to each other and to the substrate by sintering. During sintering, the film of particles are fused together, and may also become fused to the substrate, by partial melting that occurs at the high sintering temperatures that are generally between about 400.degree. C. and 2,000.degree. C. depending upon the particles being deposited. This process is disclosed, for instance, in U.S. Pat. No. 3,926,799. In practice, organic additives are added to the inorganic film before the drying step. The function of the organic additive is to bind the inorganic film particles together during drying. When an organic additive is not utilized, cracking of the inorganic film occurs during drying which results in an unacceptable membrane. The organic additive is evaporated or decomposed during the sintering step.
Examples of utilizing organic binding chemicals are disclosed in Canadian Pat. No. 1,140,003 and French Pat. No. 2,502,508.
In a second process, no further treatment is effected after the particles have been deposited on the substrate. The film particles are held together by relatively weak forces such Van der Waals, or hydrogen bonding. These films generally cannot be dried without cracking. Accordingly, they are always maintained wet. They are less able to withstand chemicals such as acids, bases, salts and solvents as compared to the membranes produced by the above-described first process. Examples of this second process are disclosed in U.S. Pat. Nos. 3,331,772; 3,413,219 and 4,077,885.
It would be desirable to provide inorganic ultrafiltration or microporous membranes capable of being dried without cracking and capable of withstanding acids, bases, salts and solvents commonly encountered in filtration processes. Furthermore, it would be desirable to provide such an inorganic membrane which does not require a high temperature sintering process and therefore does not require matching of film substrate materials having similar thermal expansion coefficients. This would permit producing membranes from a wider range of materials as compared to current processes. Furthermore, it would be desirable to provide such a process which eliminates the need for organic additives.